This invention relates generally to the display screen of a cathode ray tube (CRT) and is particularly directed to an antistatic/antireflective coating disposed on the outer surface of a CRT""s display screen.
Self-emitting display devices, such as of the CRT type, provide a video image by the bombardment of phosphor elements disposed on the inner surface of the device""s display screen by high energy electrons. The phosphor elements are generally of three types in producing the primary colors of red, green, and blue in providing a color video image on the display screen. For optimum color purity, the three groups of color phosphors should emit light characterized by three discrete spectra, with no overlap between adjacent spectral components. In practice, however, there is always some overlap between adjacent spectral components. However, by minimizing adjacent color overlap, color contrast is improved to provide a more visually pleasing video image. A high level of contrast is normally obtained by reducing the light transmission characteristic of the coating.
There are currently two approaches to increasing the color contrast of a video image on the display screen of a CRT. One approach involves the addition of organic dyes in a surface coating disposed in the form of a thin layer in the outer surface of the CRT""s display screen. The other approach to increasing video image color contrast is through the use of inorganic pigments in the display screen""s outer surface coating. Unfortunately, inorganic pigments are characterized as having only limited solubility and are difficult to disburse in organic solvents. Thus, the use of organic dyes in the display screen""s surface coating is the more commonly used approach for increasing color video image contrast.
U.S. Pat. No. 4,987,338 to Itou et al. discloses an antistatic/antiglare coating in the form of a single layer to which an organic dye is added. This approach is not particularly desirable because the organic dye is subject to bleaching out of the coating when the face plate is wiped with a wet cloth containing either water or alcohol. U.S. Pat. No. 5,291,097 to color Kawamura et al teaches the addition of an organic dye to the inner antistatic layer of a double-layer layer antireflective/antistatic coating on the outer surface of the CRT""s display screen. The inner antistatic layer containing the organic dye is then covered with the outer antireflective layer. Even with the inner antistatic layer covered by a protective outer antireflective layer, the organic dye has been observed to diffuse outwardly from the inner antistatic layer and through the outer antireflective layer with the organic dye eventually bleaching out of the antireflective/antistatic coating on the faceplate through repeated wiping of the faceplate with either a dry or wet cloth. This results in undesirable water marks on the surface. Washing out of the dye from the display screen""s outer surface coating is a particularly serious problem in a high temperature and high humidity environment.
In the past, a silane coupling agent has been added to the antireflective solution to bond to the dye and prevent its bleaching out. This approach is disclosed in U.S. Pat. No. 5,523,114 wherein one end of the silane coupling agent links to the dye and its other end links to SO2. Unfortunately, the large amounts of silane coupling agent required to reduce washing out of the dye also tends to weaken the mechanical strength of the coating and change its light refractive index. In addition, large amounts of silane coupling agent lower the coating""s electrical conductivity due to the relatively high content of nonconductive compounds. In some cases, a water soluble polymer having a structure similar to that of a dye as well as to that of a silane coupling agent is added to the antireflective coating. The dye is stabilized by linking the Si end of the polymer with the SiO2 in the antireflective coating.
The present invention addresses the aforementioned problems encountered in the prior art by providing an outer layer surface coating for the outer surface of a CRT display screen which includes a first silane coupling agent which reacts with the silica in the glass display screen to provide a high degree of adherence and a second silane coupling agent which is hydrophobic for preventing moisture from permeating into the coating layer at high humidities. This combination of silane coupling agents provides the display screen""s surface coating with a high degree of hardness, increased electrical conductivity, and reduced reflectivity.
Accordingly, it is an object of the present invention to provide an improved antistatic and/or antireflective coating for the outer surface of the display screen of a cathode ray tube (CRT) which provides a high level of color video image contrast without reducing the mechanical strength, changing the refractive index, or increasing the reflectivity of the display screen coating.
It is another object of the present invention to provide an antistatic and/or antireflective coating for a CRT display screen which retains its mechanical properties and stability at high temperatures and humidity levels, transmits reduced light for improved video image contrast, and maintains high electrical conductivity for effective electrostatic discharge.
A further object of the present invention is to improve the mechanical strength, abrasion resistance, and optical characteristics of an antistatic and/or antireflective coating for the surface of a display screen of a self-emitting color display device by adding reduced amounts of silane coupling agents to the coating.
The present invention contemplates for use on an outer surface of a glass faceplate of a self-emitting display device, wherein the glass faceplate includes a phosphor coating on an inner surface thereof, and wherein the phosphor coating is responsive to energetic electrons incident thereon for providing light for presentation of a video image on the glass faceplate, a coating comprising: an antireflective layer disposed on the outer surface of the faceplate; an organic dye disposed in the antireflective layer for increasing color purity and contrast of the video image presented on the glass faceplate; a first binding agent disposed in the antireflective layer for bonding to the dye and preventing diffusion of the dye out of the antireflective layer; and a second binding agent disposed in and bonded to the antireflective layer for providing the antireflective layer with increased water resistance for preventing washing out of the dye from the antireflective layer.